The present invention relates to the field of vehicle power steering systems, and more particularly to a power steering system having a power fluid pump, a hydraulic cylinder for moving the steering wheels of the vehicle and a fluid controller between the pump and the hydraulic cylinder for controlling the flow of pressurized fluid therebetween. More specifically, the present invention relates to a power steering system which is associated with a hydrualic system for controlling an implement on the vehicle.
Hydraulic systems which utilize a single fluid pump to supply fluid to a steering mechanism, as well as to an implement, are known, and primarily such systems have been constructed so that the steering function has priority over the operation of the implement. Priority valves for use in such combined systems are known. Such priority valves normally operate in response to a demand from the steering control to restrict the flow of fluid to the implement.
In U.S. Pat. No. 3,834,278, a combined steering and implement actuating hydraulic system is disclosed. In that patent, the fluid controller is constructed so that flow to the implement is through the steering controller. Specifically, the fluid controller in the patent is of a type which utilizes a valve which moves in response to the steering action. As the valve moves in response to the steering action, the fluid flow to the auxiliary circuit, such as to the implement, pinches off as the valve increasingly opens the fluid flow path to the steering circuit. Consequently, the main power steering system always has priority over the auxiliary circuit so far as the pressurized fluid being supplied by the power fluid pump is concerned. Conequently, the operator has optimum control of the power steering system, regardless of whether the auxiliary circuit is being actuated simultaneously, since in all circumstances movement of the valving mechanism from its neutral position can pinch off flow to the auxiliary circuit and the degree of pinching off is proportional to the distance by which the valve mechanism moves from its neutral position.
In the system disclosed in U.S. Pat. No. 3,834,278 no flow of fluid to the implement or auxiliary circuit can occur when the steering valve is in a position completely blocking off flow of fluid to the implement. Accordingly, that system when applied to a farm tractor, for example, would operate so that when turning at the end of a field, the tractor wheels could be in a full-travel position and the steering valve would be in a fully-actuated position it would not be possible to lift an implement on the tractor while making the turn.